1. Field of the Invention
The present invention relates in general to a chemical vapor deposition (CVD) depositing thin-films, in a vapor phase reaction, of a highly dielectric or ferroelectric substance such as barium/strontium titanate on a substrate such as a semiconductor wafer.
2. Description of the Related Art
In recent years, there has been a quantum jump in circuit density of integrated circuit devices produced by the semiconductor industries, and intense development activities are underway in anticipation of giga-bit order DRAMs replacing the prevailing mega-bit order DRAMs of today. Dielectric thin-film materials used to make high capacitance devices necessary for producing DRAMs have, in the past, included silicon oxide or silicon nitride films with a dielectric constant of less than 10, and tantalum pentaoxide (Ta.sub.2 O.sub.5) films with a dielectric constant of less than 20. However, newer materials such as barium titanate (BaTiO.sub.3), strontium titanate (SrTiO.sub.3) or barium/strontium titanate ((BA,Sr)TiO.sub.3) appear to be more promising.
When growing such a metal oxide thin-film on a substrate in a vapor phase chemical reaction, the substrate is placed on a susceptor disposed in a vacuum-tight film deposition chamber, and the substrate is heated, by such means as a heater enclosed in the susceptor, to a specific temperature, and a mixture of a feed gas and a reactant gas (oxygen-containing) is ejected onto the substrate through a gas showering head.
To produce a thin-film of a uniform quality using such a method, careful consideration must be given to managing the process of flowing the gaseous mixture over the substrate, which means that not only does the routing pattern from the gas showering head to the substrate become an important parameter, but also the overall flow process, including the effect of the spent gas exhaust path in the film deposition chamber.
For example, it may seem desirable to place a spent gas exhaust opening directly below a pedestal which supports the substrate so as to form an axisymmetric pattern of flow of the film forming gas enveloping the substrate. However, such an arrangement is not only difficult to do, because of the presence of other components associated with the pedestal such as a pedestal support shaft and an elevating device, but it increases the length of the flow path within the chamber, leading to a problem of precipitation and adhesion of reaction products that could become a potential source of contamination in the deposited films. On the other hand, an arrangement may be considered in which a plurality of exhaust openings are provided symmetrically around the outer periphery of the substrate holder to produce a symmetrical flow pattern of the film forming gas over the substrate, but such a design would lead to an expanded apparatus requiring a large floor space, and also would lead to a difficulty of mechanical interference with the exhaust openings and a substrate transport exit opening.
Therefore, it is a first object of the present invention to provide a compact thin-film deposition apparatus which promotes stable growth of a high quality thin-film product of uniform quality.
Furthermore, one of the characteristics of a feed material is that after the feed liquid is vaporized, the range of stability of the feed vapors is extremely narrow such that the vapors are condensed when the ambient temperature is low and the vapors are decomposed when the ambient temperature is high. The present inventors had proposed a method in a Japanese Patent Application, Laid-Open Publication H9-2896, to counter the difficulties posed by such a narrow stability range of vapors by controlling the inner wall temperature of the deposition chamber at a certain temperature close to the vaporization temperature. It has been found that, although it is possible to effectively suppress the formation of reaction products, such products cannot be eliminated completely and fine particles are produced and become attached to the inner walls of the deposition chamber.
In the course of conducting thermal analyses of the feed materials, it was found that precursors of each of the liquid feed materials for BST or SBT, for example, do not have a complete and clear temperature range where all three materials can remain in a stable gaseous state, resulting in condensation or decomposition of at least a portion of the feed vapor. It was also discovered that condensed and adhered products are easy to wash off when compared with decomposed and adhered products.
Therefore, it is a second object of the present invention to provide a deposition apparatus which produces a high quality and uniform thin-film product while avoiding decomposition of the feed vapor at high temperatures.